Dual-circuit steer-by-wire steering system comprising a common cradle

ABSTRACT

The invention relates to a steer-by-wire steering system comprising a hydraulic control unit that acts on the steerable wheels ( 1, 20 ), the control unit having at least one cradle ( 4, 23 ), which is displaceably mounted on at least one guide rod ( 5, 24 ) that passes through the respective cradle ( 4, 23 ). The respective cradle ( 4, 23 ) and the corresponding guide rod(s) ( 5, 24 ) form working chambers (A 11 , A 12 , A 21 , A 22 ) of a piston-cylinder system, whereby each respective guide rod ( 5, 24 ) has at least one channel (KA 11 , KA 12 , KA 21 , KA 22 ), each terminating in the vicinity of a working chamber (A 11 , A 12 , A 21 , A 22 ). The cradle or cradles ( 4, 23 ) adjust the steerable wheels ( 1, 20 ) directly or by means of steering tie rods ( 2, 21 ).

BACKGROUND OF THE INVENTION

The present invention relates to a steer-by-wire steering system,comprising a hydraulic control unit that acts on the steerable wheels.

The control units of generic dual-circuit steer-by-wire steering systemsnormally consist of two control cylinders, which are individuallymounted on their own and which need flexible feed lines to supply thefluid medium. Due to their structure the control cylinders are normallybulky and prone to fault on account of the flexible feed lines.

Object of the present invention is to create a steer-by-wire steeringsystem, the structure of which is as compact and as non-prone to faultas possible.

SUMMARY OF THE INVENTION

This object is achieved according to the invention with a steer-by-wiresteering system wherein the control unit has at least one cradle, whichis adapted to adjust the steerable wheels directly or via tie rods,wherein the at least one cradle is displaceably mounted on at least oneguide rod, which in particular passes through a respective one of the atleast one cradle, which, together with the at least one pertaining guiderod, form working chambers of a piston-cylinder system, wherein the atleast one guide rod is a tube and has at least one channel, whichrespectively ends in a vicinity of one of the working chambers, whereina first channel is formed by the tube itself, wherein an inner tube isdisposed in the first channel and has an outer diameter that is lessthan the inner diameter of the first channel, and wherein the inner tubeforms a second channel.

The steer-by-wire steering system in accordance with the invention isdistinguished in that as a result of fixed guide rods, which inparticular are secured directly to the vehicle bodywork or valve blocks,a large degree of flexural rigidity of the steering system is obtained,since the guide rods employed have a greater diameter in relation to thepiston rod used in conventional control cylinders. A cradle isdisplaceably mounted on the guide rods of the steer-by-wire steeringsystem in accordance with the invention, whereby the cradle as well asthe guide rods form working chambers of a piston-cylinder system. It iswithin the sense of the invention to use only one guide rod or howeveralso several guide rods for the steering system in accordance with theinvention. It is likewise within the sense of the invention that allguide rods form piston-cylinder systems with the cradle or cradles.However it is also possible that a minimum of one guide rod is provided,which only fulfills the cradle-guiding function, but does not form apiston-cylinder system.

Generally, the arrangement of the piston guide rods on the motor vehicleis arbitrary. Advantageously, the guide rods are arranged transversallyto the driving direction and parallel to the roadway. However, it isalso completely possible to arrange the guide rods in another directionon the vehicle.

If more than one guide rod is present, these are preferably arrangedparallel to each other at the same height. However, it is also possiblethat the guide rods are arranged one above the other and parallel toeach other. The point of application of force of the steering tie rodson the cradle(s), which is/are displaceably mounted on the guide rods,is to be selected as a function of the given installation space.Advantageously, the points of application of force of the steering tierods of both guided wheels are to be selected in such a way that theylie centrally between two parallel guide rods, so that no torque actsupon the guide rods. As a result of this central application of force itis in particular possible to provide long guide rods, whichadvantageously leads to minimum steering error when the springs arecompressed and released. One or more long guide rods can also be used,whereby in addition a central steering tie rod links the two points ofapplication of force of the two steering tie rods with one another. Thiscentral steering tie rod is to be arranged parallel to the guide rod(s).Advantageously, it is displaceably mounted on the bodywork parallel tothe guide rod(s), whereby advantageously no flexural stress acts uponthe steering tie rods.

Because the link with the vehicle framework occurs far on the outsidedue to the long guide rods, a more rigid connection of the guiderod-cradle system arises. Advantageously, the steer-by-wire steeringsystem in accordance with the invention has a modular construction.Thus, it is possible to place the guide rods between two valve blocks.Depending on the type of motor vehicle, the valve blocks and/or theguide rod-cradle system can be selected accordingly, whereby standardmodules can be used for different types of motor vehicle.

Because the guide rods themselves form the line for the fluid medium,advantageously no additional connecting hoses are necessary as in thecase of conventional steering systems, so that advantageously less riskof leakage results.

If two guide rods are used, each of which on their own form feed linesto working chambers of piston-cylinder systems, each guide rod is initself a component of a steering circuit, whereby the two steeringcircuits can be implemented purely as a dual-circuit or in addition, insome case as a mono-circuit, that is to say for example with a commonpump. In principle, the steer-by-wire steering system in accordance withthe invention can be wired similarly to all known steering circuitwiring. The two steering circuits, for example, can possess valveshaving open or closed center. However, it is also possible that asteering circuit is dimensioned as far as the level of force isconcerned, in such a way that it alone can apply the steering force whenthe motor vehicle is being driven in normal operation. The secondsteering circuit then serves as a reserve, which is activated either inthe event of failure of the first steering circuit or however ifnecessary, for example in parking and/or shunting operation, when highsteering force is necessary.

As already stated, if the above-described steer-by-wire steering systemis used in this way, the control valves fitted in the steering circuitscan be valves having open or closed center. The use of valves havingclosed center with simultaneous use of an accumulator has the advantagethat the pump does not have to be in continuous operation in order tobuild up a working pressure of the fluid medium, whereby advantageouslyenergy and thus fuel are saved.

As also already stated, the cradle in the case of the steer-by-wiresteering system in accordance with the invention can be configured bothwith tandem and single cylinders. In the case of the tandem cylinder,the working chambers of the piston-cylinder systems of each steeringcircuit are arranged beside each other in the axial direction and areseparated from one another by gaskets. If only one guide rod and onecradle and/or single cylinder is used, the working chambers of bothsteering circuits are arranged beside each other and/or one behind theother in the axial direction. Therefore a relatively small dimensionalsize results, whereby however the cradle and/or the control cylinder,which encompasses the guide rod from outside, is longer than the controlcylinders of the tandem system.

Brief Description of the Drawings

Embodiments of the steer-by-wire steering system in accordance with theinvention are described in detail below with reference to figures.

There is shown:

FIG. 1 a schematic illustration of a steer-by-wire steering system inaccordance with the invention;

FIG. 2 a dual-circuit steer-by-wire steering system with centrallyacting steering tie rods;

FIG. 3 a sectional view through a steer-by-wire steering system inaccordance with FIG. 2;

FIG. 4 a perspective view of a steer-by-wire steering system inaccordance with the invention;

FIG. 5 a view of the feed lines of the dual-circuit steer-by-wiresteering system in accordance with FIGS. 1 to 4;

FIG. 6 a detailed illustration of a steer-by-wire steering system inaccordance with the invention with only one guide rod;

FIG. 7 a cross sectional view through the piston-cylinder system of thesteer-by-wire steering system in accordance with FIG. 6.

Description of Specific Embodiments

FIG. 1 shows a schematic illustration of a first embodiment of thesteer-by-wire steering system in accordance with the invention, wherebya cradle 4, which is movably and/or displaceably mounted on two guiderods 5, adjusts steerable wheels 1 by means of steering arms 3 andsteering tie rods 2. The cradle 4 together with the guide rods 5 formsfour working chambers A₁₁, A₁₂, A₂₁, and A₂₂. The guide rods 5 arerigidly mounted on the bodywork or chassis of the vehicle between twovalve blocks 6. The cradle 4 and the guide rods 5 form a specialpiston-cylinder system in which the kinetics are reversed, since herethe piston and/or the guide rod(s) are static and the cylinder and/orcradle moves.

FIG. 2 shows a further embodiment of the steer-by-wire steering systemin accordance with the invention, in which the points of application offorce 2 a of the steering tie rods 2 are arranged on the cradle 4centrally between the two guide rods 5. As a result of the centralarrangement of the points of application of force 2 a to the guide rods5 minimum stress on the cradle 4 arises, as a result of which theconstruction of this embodiment does not need to be so substantial.

FIG. 3 shows a cross sectional view of the piston-cylinder system inaccordance with the embodiment of FIG. 2. The guide rods 5 ₁ and 5 ₂ aremounted between the valve blocks 6 ₁ and 6 ₂. The cradle 4 comprises twocylinders 4 a and 4 b, which are rigidly connected with one another bymeans of intermediate parts 4 c. The guide rods 5 ₁ and 5 ₂ are placedbetween the valve blocks 6 ₁ and 6 ₂. The free ends 5′₁ and 5″₁, 5′₂ and5″₂ are secured by means of fastening parts 6 a to the valve blocks 6 ₁and 6 ₂, in particular with their ends screwed into the valve blocks.The guide rods 5 ₁ and 5 ₂ are formed as tubes, which advantageouslyhave a circular cross section for outside and inside diameters. However,it is also possible to select any cross-sectional shape. Nevertheless,it is to be ensured here that a possible simple and durable seal must beguaranteed between the guide rods and the cradle. By selecting acircular cross section it is possible that the cavity of each cylinder 4a, 4 b can be produced for example by means of a borehole or however isformed by a simple tube. The guide rods 5 ₁ and 5 ₂ also have continuouscavities. The cavities can be realized by means of boreholes. However,it is equally possible to use a tube as a guide rod.

In each case, an inner tube 18 ₁, 18 ₂ is inserted from one end in thecavity of each guide rod 5 ₁, 5 ₂. The end 5″₁ and 5″₂ of each guide rod5 ₁, 5 ₂ is closed in each case by means of a seal D₁ and/or D₂, so thatthe medium flowing through the inner tube 18 ₁ and/or 18 ₂ inside theguide rod 5 ₁ and/or 5 ₂ into the respective working chamber A₁₂ and/orA₂₂ cannot escape from the guide rod 5 ₁ and/or 5 ₂ or from thepiston-cylinder system. The inner tubes 18 ₁ and/or 18 ₂ in each case ontheir end inside the cavity are encompassed by a seal D, which seals theinner tube 18 ₁ and/or 18 ₂ against the inner wall of the guide rod 5 ₁and/or 5 ₂ and thus divides the interior of the guide rods 5 ₁ and/or 5₂ into two sub-areas. The guide rod has window-like orifices 7, 8, 9 and10, which connect the feed and/or extraction lines with the workingchambers A₁₁, A₁₂, A₂₁ and A₂₂.

Should the cradle 4 be moved from left to right, a pressure can be builtup either in the working chamber A₁₁ or in the working chamber A₂₂.Thus, it is within the sense of the invention to apply fluid pressure toboth cylinders at the same time. Likewise, it is within the sense of theinvention, and this also applies for all embodiments illustrated, thatthe cylinder 4 a together with the guide rod 5 ₁ is a component of afirst steering circuit and the cylinder 4 b as well as the guide rod 5 ₂is a component of a second steering circuit. As already described, bothsteering circuits can operate in parallel. However, it is also possiblethat the first steering circuit is provided for normal driving and isdimensioned accordingly as far as its level of force is concerned,whereby the second steering circuit is only activated to assist thefirst steering circuit, for example during parking and/or shuntingoperation. If the first steering circuit should fail, the secondsteering circuit provides a fall back level, as a result of which thevehicle can continue to be steered safely.

FIG. 4 shows a perspective view of a steering system in accordance withthe invention with two guide rods 5 arranged in parallel, on which twocylinders are rigidly connected with one another by means ofintermediate parts 4 c, which together form a cradle 4. The guide rods 5are placed between two valve blocks 6. The valve blocks 6 correspond tothe valve blocks 6 ₁ and 6 ₂ schematically illustrated in FIG. 3.

With FIG. 5 once again the specific structure of the piston-cylindersystem is described, whereby the general structure corresponds to thatof FIG. 3, the difference being that the cylinders 4 are connecteddirectly, (shown by 4 d). that is to say without intermediate elements(4 c). The guide rods 5 are formed as tubes, which are held parallel toeach other by means of fastening parts 15, 16. The free ends 5″₁, 5″₂are closed by means of screw plugs 12, 14 and inlaid gaskets 11, 13, sothat no fluid medium can escape at these ends. The screw plugs 12 and/or14 press the guide rods 5 ₁ and 5 ₂ with their free ends 5′₁, 5′₂ intothe respective cylinder block. As a result, a sure and durableconnection between the guide rods with their channels and the valveblocks results. It is, however, equally possible in an alternativeembodiment to screw the ends 5′₁ and 5′₂ of the guide rods with thevalve blocks. The fluid medium is pumped through the opposite free endsof the guide rods 5 ₁ and 5 ₂ into the channels KA₁₁ and KA₂₁ formed bythe guide rods 5 ₁ and 5 ₂ towards the working chambers A₁₁ and A₂₁and/or let out from these. The channels KA₁₁ and KA₂₁ are formed by theinner wall of the guide rod 5 ₁ and/or 5 ₂ and the inner tube 18 ₁and/or 18 ₂. The channels KA₁₁ and KA₂₁ are connected via window-likeorifices 8, 9 with the associated working chambers A₁₁ and A₂₁. Thechannels KA₁₂ and KA₂₂ are formed by the inner tubes 18 ₁ and/or 18 ₂.The channels KA₁₁ and KA₁₂ or KA₂₁ and KA₂₂ in each case are sealed by aseal D, which encompasses or spans the final area of the inner tube 18 ₁and/or 18 ₂ and lies against its outer wall and at the same time againstthe inner wall of the guide rod 5 ₁ and/or 5 ₂. The channels KA₁₂ andKA₂₂ are connected with the associated working chambers A₁₂ and A₂₂ viawindow-like orifices 7, 10. The ratio between inner diameter R₁ of theguide rods 5 ₁ and/or 5 ₂ and outer diameter R_(A) of the inner tubes 18₁ and/or 18 ₂ is to be dimensioned such that the flow cross-sections areroughly the same size or are suitable for the respective demands. Thecylinders 4 lie with their end areas 4 e, 4 f sealingly outwards on theguide rod 5 ₁ and/or 5 ₂, so that no fluid medium can escape from theworking chambers into the environment. The working chambers A₁₁ and A₁₂or A₂₁ and A₂₂ of the first and second steering circuit in each case areseparated from one another by a circular part K, which in particular canbe molded in the shape of a collar outside on the guide rod 5 ₁ and/or 5₂. So that the inner tube 18 ₁ and/or 18 ₂ cannot be pushed too far intothe guide rod 5 ₁ and/or 5 ₂ stop elements S are to be provided. Thestop elements S in particular can be inserted or screwed into awindow-like orifice and/or borehole of the guide rods 5 ₁ and/or 5 ₂.

FIG. 6 shows an alternative embodiment of a dual-circuit steer-by-wiresteering system in accordance with the invention, whereby only onesteered wheel 20 is illustrated, which can be moved by means of asteering tie rod 21, which is linked via the joint 21 a to a centralguide rod 26. The central guide rod 26 is rigidly connected with acradle 23, which in turn is displaceably mounted on a guide rod 24. Theguide rod 24 is mounted between two valve blocks 25, which are connectedwith the vehicle chassis 22. The central steering tie rod 26 isdisplaceably mounted parallel to the guide rod 24 by means of slidingbearings 27 on the chassis 22. As a result of the oblong arrangement ofthe cradle 23 the guide rod 24, which is smaller as far as its diameteris concerned, is relatively resistant to bending, so that the fasteningpoints of the guide rod 24 can be arranged relatively far on the outsideand therefore directly on the chassis 22. FIG. 7 shows thepiston-cylinder system in accordance with the embodiment of FIG. 6 incross section. The guide rod 24 with its two ends lies in two valveblocks 25 whereby fluid medium can flow into the channels KA₁₁, KA₁₂,KA₂₁ and KA₂₂ through the valve blocks 40 and/or flow out from these.Two inner tubes 28 ₁ and 28 ₂ are each inserted from one end of the tube24 into this and lie with their ends inserted in the tube 24 againststop elements 31, 32. The free ends of the inner tubes 28 ₁ and 28 ₂ areencompassed by circular gaskets 35, 36, which lie sealingly against theouter wall of the inner tubes 28 ₁, 28 ₂ and the inner wall of the tube24. These gaskets separate the channels of one steering circuitrespectively from one another. The channels KA₁₂ and KA₂₂ are separatedsealingly from one another by a gasket 39 inlaid in the tube 24. Theguide rod and/or the tube 24 have window-like orifices 33, 34, 41 and42, through which the channels KA₁₁, KA₁₂, KA₂₁ and KA₂₂ are connectedwith the working chambers A₁₁, A₁₂, A₂₁ and A₂₂. The displaceablymounted cylinder and/or cradle 23 encompasses the guide rod and/or thetube 24 and with this together forms the working chambers. The ends ofthe cylinder 23 are closed by means of gaskets 38, so that no fluidmedium can escape from the working chambers A₁₁ and A₂₁ into theenvironment. The working chambers A₁₂ and A₂₂ are separated from oneanother by means of a collar 25 a and associated gasket 37 arranged onthe cylinder 23. The working chambers A₁₁ and A₁₂ are separated from oneanother by a circular and/or collar-shaped part 29 and associatedgasket. For assembly, the part 29 can be pushed from the end onto thetube 24 and can be then connected captively with the tube 24. However,it is also possible to mold this collar-shaped part on the tube 24.Likewise, the working chambers A₂₁ and A₂₂ are separated from oneanother by means of a separating element 30 and associated gasket. Theseparating elements 29, 30 are rigidly fastened to the tube 24, whereasthe partition 25 a is displaced with the cylinder 23.

Of course it is possible that the parts forming the cylinders are notproduced integrally, for example from a tube and/or a cylinder, but areformed from two or several parts. Thus, it is possible, for example, toform the cylinder by means of two half shells, whereby gaskets are to beprovided between the two parts.

Generally, the connection of the channels formed by the guide rods andinner tubes on the fluid circuit of the respective steering circuits,can be configured arbitrarily. Of course, it is possible to deviate fromthe embodiments described specifically and yet make use of the inventiveidea. Thus, a guide rod can also be formed from two or several parts, ifthis is necessary for easier assembly. The individual parts forming aguide rod can for example be inserted into one another or screwedtogether or fastened or connected with one another in any other way. Thesame applies for the parts forming the cylinders and the cradle as awhole.

The specification incorporates by reference the disclosure of PCT/EP02/13726 filed Dec. 4, 2002 and German priority document 101 607 64.4 filedDec. 11, 2001.

The present invention is, of course, in no way restricted to thespecific disclosure of the specification and drawings, but alsoencompasses any modifications within the scope of the appended claims.

1. A steer-by-wire system comprising: a hydraulic control unit that actson steerable wheels 1, wherein said control unit has at least one cradlewhich is adapted to adjust said steerable wheels directly or via tierods; at least one guide rod, wherein said at least one cradle isdisplacebly mounted on said at least one guide rod, which in particularpasses through a respective one of said cradles, wherein said at leastone cradle together with said at least one pertaining guide rod ,formworking chambers of a piston-cylinder system, wherein said at least oneguide rod is a tube and has at least one channel which respectively endsin a vicinity of one of said working chambers, wherein a first channelis formed by said tube itself; and an inner tube disposed in said firstchannel, wherein said inner tube has an outer diameter that is less thanan inner diameter of said first channel, and wherein said inner tubeforms a second channel.
 2. A steer-by-wire steering system according toclaim 1, wherein a plurality of cradles are provided that are rigidlyconnected with one another directly or by means of at least oneintermediate part and form a single cradle.
 3. A steer-by-wire steeringsystem according to claim 1, wherein a given one of said at least oneguide rod is provided with a plurality of channels that extend from oneor both end faces or regions of said guide rod into said guide rod.
 4. Asteer-by-wire steering system according to claim 1, wherein said atleast one guide rod is secured or mounted on a vehicle chassis directlyor by means of intermediate parts.
 5. A steer-by-wire steering systemaccording to claim 1, wherein said at least one guide rod is disposedbetween two valve blocks and wherein at least one of said channels andfeed lines of said at least one guide rod are in operative communicationwith valves of at least one of said valve blocks.
 6. A steer-by-wiresteering system according to claim 5, wherein two guide rods aredisposed parallel to one another between two valve blocks and passthrough a cradle wherein respective channels of said two guide rods arerespectively in operative communication with only one valve block,wherein said cradle with each guide rod, forms working chambers of apiston-cylinder system, and wherein each guide rod, together with theworking chambers formed thereby and the associated valve block, form aseparate steering circuit.
 7. A steer-by-wire steering system accordingto claim 6, wherein a respective guide rod is surrounded by a cylinderend faces of which respectively rest in a sealing manner againstpertaining ones of said guide rods, and wherein two cylinders areprovided and are interconnected directly or via at least one connectionpart and form said cradle.
 8. A steer-by-wire steering system accordingto claim 6, wherein points of application of force or mounting locationsof said tie rods are disposed between said two guide rods in particularbeing centrally disposed relative to said guide rods.
 9. A steer-by-wiresteering system according to claim 6, wherein points of application offorce or mounting locations of said tie rods are disposed adjacent tosaid two guide rods.
 10. A steer-by-wire steering system according toclaim 6, wherein said at least one guide rod is a tube having an innerdiameter that forms said first channel and in an interior of which isdisposed said inner tube having an outer diameter that forms said secondchannel, wherein both of said channels are operatively connected withthe same valve block, and wherein a gasket sealingly engages about asurface of said inner tube in a vicinity of a free end thereof andsealingly rests against an inner wall of said first channel of saidguide rod.
 11. A steer-by-wire steering system according to claim 10,wherein said gasket separates said first and second channels from oneanother and divides an interior of said guide rod into two regions. 12.A steer-by-wire steering system according to claim 11, wherein at leastone window-like orifice is disposed in each wall of said guide rod andrespectively connects a region with a working chamber.
 13. Asteer-by-wire steering system according to claim 11, wherein in avicinity of said gasket said guide rod is provided with a peripheral,collar-shaped projection that separates the working chambers from oneanother.
 14. A steer-by-wire steering system according to claim 1,wherein a guide rod that together with a carriage form working chambersis provided with at least one outwardly directed collar that inparticular is formed thereon, wherein a given collar respectivelyseparates two working chambers from one another.
 15. A steer-by-wiresteering system according to claim 1, wherein a guide rod formed by anouter tube passes through said cradle wherein disposed in said outertube are two inner tubes that respectively extend from an end face intosaid outer tube, wherein a gasket divides an interior of said outer tubeinto two partial chambers that are separated from one another, and arespective annular gasket which is disposed between an inner tube andsaid outer tube, seals the two channels formed by said outer tube andsaid inner tube from one another, and wherein said outer tube isprovided with window-like orifices that connect the channels with theworking chambers that are formed by said outer tube and said cradle. 16.A steer-by-wire steering system according to claim 15, wherein saidcradle is provided with a tubular part through which said outer tubepasses, and wherein gaskets disposed between said tubular part and saidouter tube seal the working chambers toward the outside.
 17. Asteer-by-wire steering system according to claim 15, wherein arespective collar which is disposed on said outer tube and is directedoutwardly, separates said working chambers of a steering circuit fromone another.
 18. A steer-by-wire steering system according to claim 17,wherein said collar is formed on said outer tube or is formed by anadditional annular part that is connected with said outer tube.
 19. Asteer-by-wire steering system according to claim 15, wherein an annulargasket and an inwardly directly collar that is disposed on a tubularportion of said cradle, separate two adjacent working chambers of twosteering circuits from one another.
 20. A steer-by-wire steering systemaccording to claim 1, wherein an end of respective ones of said tie rodsare rotatably mounted on said cradle.
 21. A steer-by-wire steeringsystem according to claim 1, wherein a central tie rod is disposedparallel to said cradle and is secured thereto or integrated therewith,wherein said central tie rod is displaceably mounted on a vehiclechassis parallel to said cradle, and wherein an end of respective onesof said tie rods are rotatably or pivotably mounted on said central tierod.
 22. A steer-by-wire steering system according to claim 1, whereintwo steering circuits operate in parallel at the same time, and inparticular an associated control valve is a valve having open center orclosed center.
 23. A steer-by-wire steering system according to claim22, wherein a second steering circuit has a level of force that is thesame or less than that of a first steering circuit.
 24. A steer-by-wiresteering system according to claim 1, wherein a level of force of afirst steering circuit is dimensioned such that, with the exception of aparking or shunting operation of a vehicle being steered, appliessufficiently large forces for a normal driving operation.
 25. Asteer-by-wire steering system according to claim 1, wherein a secondsteering circuit takes over the function of the first steering circuitupon failure of said first steering circuit.
 26. A steer-by-wiresteering system according to claim 25, wherein a control valve of saidfirst steering circuit is a valve having open center, and a controlvalve of said second steering circuit is a valve having closed center.27. A steer-by-wire steering system according to claim 1, wherein saidsteering system is provided with additional guides for a mounting ofsaid at least one cradle.
 28. A steer-by-wire steering system accordingto claim 27, wherein at least one additional guide is a guide rod thatpasses through said cradle.